1. Field of the Invention
The present invention relates to an alkaline battery that does not include mercury, i.e. is devoid thereof. More specifically, the invention relates to an alkaline battery with improved mixing and dispersion of zinc powder and an effective metal in an alkaline electrolyte, whereby such battery has high vibration strength and impact resistance, and whereby it is possible to eliminate degradation of ability of the battery to discharge under heavy load and low temperature.
2. Description of the Related Art
In the conventional alkaline battery, a small amount of mercury is used for forming an amalgam for increasing corrosion resistance by elevating hydrogen overvoltage of zinc used as a primary active material for a negative electrode or cathode, and for restricting passivation of zinc by converting the same into zinc oxide.
However, since mercury is a harmful substance, there have been progressive attempts to avoid use of mercury in recent years. Among various technologies for avoiding mercury, the most advanced technology in the prior art is to add a small amount of an effective metal other than mercury, such as lead, aluminum, indium, gallium, bismuth or the like to zinc for alloying. Various studies have been made with respect to the influence and so forth of the types of materials to be added or to the amount to be added for the performance of the battery, by varying compositions with employing a variety of effective metals.
Separately from the method of alloying of zinc, there is has been another attempt for adhering an effective metal, such as lead, aluminum, indium, gallium, bismuth or the like, to zinc or zinc alloy powder. The method of adhering the effective metal to the zinc or zinc alloy has been proposed in Japanese Patent Laid-open (Kokai) Nos. 60-1775 and 60-236456. In the method proposed in such publications, the effective metal and zinc powder or zinc alloy powder are reacted for substitution in an acid solution. On the other Japanese Patent Laid-open No. 1-105466 discloses a method for adhering the effective metal by directly putting an oxide or hydroxide of the effective metal in a gel form alkaline electrolyte.
However, in case of the method by reaction for substitution, since the zinc or the zinc alloy powder put in the acid solution, in which the effective metal is previously dissolved, and then mixed therewith by stirring, there is required a substantial stirring period in order to uniformly adhere the effective metal on the surface of zinc or zinc alloy powder. Furthermore, a drying process is required after the stirring and mixing process, such that the overall process becomes complicated.
On the other hand, in case of the method by directly putting the effective metal into the gel form electrolyte and then stirring for mixing, such method also requires a substantial period for uniformly distributing the effective metal relative to zinc or zinc alloy. Furthermore, by continuing stirring for a long period of time, the hydrogel structure can be destroyed to cause a lowering of viscosity resulting in secondary agglomeration or precipitation of the powder.
As is well known, gel form negative electrodes are employed as the negative electrodes for the conventional alkaline batteries. The gel form negative electrode is typically formed by mixing and kneading zinc powder, alkaline electrolyte and a gelling agent. The gelling agent is added for maintaining the zinc powder in a uniformly distributed gel form or state, for increasing the contact surface area of the zinc powder and for improving reaction efficiency.
As the gelling agent, powder form gelling agent and granular gelling agent which can hold the electrolyte are typically used. The powder form gelling agent is typically carboxymethyl cellulose (CMC), poly acrylate or salts thereof. On the other hand, the granular gelling agent is typically cross-linking and branching type poly (meta) acrylate or salts thereof as to use solely or with the gelling agent containing the same as a primary component.
However, in case of the gel form negative electrode using the powder state gelling agent, the gel form electrolyte forming the gel form negative electrode can be caused to flow in response to a strong impact, such as that applied by the battery being dropped, the result being that uniformity of distribution of zinc powder is destroyed, causing local concentration of the zinc powder. This degrades contact between the zinc powder particles or between the zinc powder and the negative collector and thus results in lowering of performance of the battery. In the prior art, a small amount of mercury has been used for avoiding such problem. However, from the ecological viewpoint, it is highly desirable to avoid use of mercury.
On the other hand, in the case of the granular gelling agent, such granular gelling agent swells by absorbing electrolyte and fills gaps between the zinc powder particles to restrict movement of the zinc powder particles upon exertion of vibration or impact. This is effective for preventing lowering the battery performance due to lowering of uniformity of zinc distribution. On the other hand, even when discharging of the battery is processed, the electrolyte absorbed in the granular gelling agent tends to be held therein. This creates a problem in that degradation of heavy load discharge performance and low temperature discharge performance cannot be satisfactorily eliminated.
In order to solve the problems set forth above, various attempts, such as use of powder state gelling agent and the granular gelling agent in combination or so forth, have been made. However, no such attempt has successfully achieved a satisfactory improvement.